1. Field of the Invention
The present invention relates generally to A/D (analog-to-digital) converter circuits and more particularly to an A/D converter circuit for a sampling signal of variable frequency.
2. Description of the Prior Art
There has been proposed so far an apparatus in which a video signal, for example, is converted into a digital signal and the digital signal is processed to effect so-called line frequency doubling. This previously-proposed apparatus is disclosed in Japanese laid-open patent application Ser. No. 61-137,481.
When such an apparatus converts, for example, a video signal in the form of analog to digital signal, if the band of a signal supplied to an A/D converter circuit is not limited to be lower than that of a half frequency signal of a sampling signal used for the A/D conversion, waveform-distortions such as a folded distortion and a sampling distortion will occur due to sampling theory, thus making it impossible to obtain an original signal of high quality.
To solve the above-mentioned problem, the following technique is known, in which a low-pass filter (LPF), referred to as a pre-filter, is provided at the preceding stage of the A/D converter circuit and a low-pass filter (LPF), referred to as a post-filter, is provided at the succeeding stage of a D/A (digital-to-analog) converter circuit and the cutoff frequencies of these low-pass filters are selected to be equal to the half frequency signal of the sampling frequency for the A/D conversion to thereby reduce the above-mentioned waveform-distortions such as a folded distortion and a sampling distortion.
In the above-mentioned conventional apparatus, in general, the sampling frequency for the A/D conversion is fixed so that the cutoff frequency of the low-pass filter might be fixed from a design standpoint. Thus, the circuits having fixed cutoff frequencies are sufficient for use.
Meanwhile, the following apparatus is proposed, in which various kinds of video signals having different line frequencies are supplied thereto and converted into digital signals, these digital signals are written in a frame memory and converted into a video signal having a desired constant line frequency such as 64 kHz for display. This previously-proposed apparatus is disclosed in our U.S. co-pending patent application Ser. No. 110,832 filed on Oct. 21, 1987. Should the sampling rate of the A/D conversion for an incoming video signal during one horizontal scanning period be made constant, the sampling frequency for the A/D conversion has to be varied as the line frequency of the incoming video signal is varied. If the cutoff frequency of the low-pass filter is fixed as earlier noted, there is a risk that the change of the sampling frequency may cause the waveform-distortions such as a folded distortion and a sampling distortion.
If the cutoff frequency of the low-pass filter is selected in consideration of the lowest sampling frequency, there is then a large risk that the quality of a reproduced picture will be deteriorated when the line frequency of the incoming video signal is high because the line frequency of the incoming video signal is varied more than twice.
In accordance with the teaching of the prior art as mentioned above, when the sampling frequency of the A/D converter circuit is varied, good conversion cannot be effected.